1. Field of the Invention
This invention relates to a wire antenna for circularly polarized wave, particularly to a wire antenna for circularly polarized wave that has a planar and thin shape as a whole.
2. Description of the Related Art
Conventionally known antennas for circularly polarized wave are a helical antenna, a cross dipole antenna, a patch antenna, etc. In recent years, the antenna for circularly polarized wave has been being required in mobile communications. There is a need for the antenna to be a shape without projections on its outside.
A patch antenna utilizing a dielectric substrate is known as an antenna serving for the above use. However, an expensive polytetrafluoroethylene (Teflon, Registered Trademark) substrate or the like is necessary in order to make the antenna thin, small and light. Therefore, the antenna satisfies requirements but is very costly. The helical antenna is a wire antenna, but is hard to be practised into a thin type. The cross dipole antenna needs two feed points, so that a power distributor and a phase shifter are indispensable, thus making its costs high.
Japanese Laid Open Patent Publication No. 61-252701 shows an art related to such antennas for circularly polarized wave.
This publication discloses an antenna in which a branch conductor of a length of about one fourth .lambda. (lambda) is arranged parallel to a loop conductor of an overall length of about one .lambda. (lambda). It generates circularly polarized waves by adjusting a gap between the conductors.
Japanese Laid Open Patent Publication No. 2-214304 shows another art related to such antennas for circularly polarized wave.
This publication discloses an antenna in which an earth plate is disposed on a rear surface of a dielectric substrate and in which a circular loop conductor is provided on a surface of the dielectric substrate. Particularly, a central conductor of a feeding coaxial line is linearly connected with an L-shaped element fitted on the loop conductor, and an outer housing conductor is linearly connected with the earth plate.
The antennas for curcularly polarized wave shown in the publications 61-252701 and 2-214304 are three-dimensional.
According to an experiment, in the antenna of 61-252701, a gap between the loop conductor and the branch conductor as well as a height from a reflecting plate are important. Unless the above dimensional accuracy is strictly kept, the antenna cannot maintain a good characteristic. Moreover, there is a practical problem in consideration of mechanical strength of a structure for standing the conductors on the reflecting plate, specially in view of vibration when mounted on a car.
On the other hand, in the antenna of 2-214304, the dielectric substrate is indispensable for its structure. Generally, the dielectric substrate of good high-frequency characteristic is expensive, so that it is difficult to decrease costs. If the antenna is constructed in a wire shape, a perturbation element must be added on a point of the loop, thus needing a soldering step or the like.